Spark ignited internal combustion engines can be fuelled with liquefied natural gas (LNG) that is delivered to the engine in a gaseous form at a relatively low pressure. This is because such engines normally inject the fuel into the intake air system (e.g. port injection) or into the combustion chamber during in the intake stroke or early in the compression stroke when the in-cylinder pressure is still relatively low.
In such low pressure engine systems, the engine can be fuelled with natural gas from the vapor space referred to herein as natural gas vapor when the pressure in the vapor space is above a predetermined threshold value. The natural gas vapor is delivered to an intake manifold where it mixes with air forming an air/fuel mixture, or charge, which is then introduced into respective combustion chambers. A minimum vapor pressure is required for adequate mixing and to ensure natural gas flow rate meets the flow rate demand of the engine. The average flow rate demand of the engine may be greater than the average evaporation rate of the LNG inside the storage vessel such that over time the vapor pressure drops below the threshold value. In this situation, operation of previous spark ignited internal combustion engines had to be suspended until the pressure in the vapor space increased beyond the threshold value. As would be known to those skilled in the technology hysteresis in the vapor pressure threshold could be employed to reduce the flip-flopping between operational and non-operational engine modes. The hysteresis increases the time required for vapor pressure build up delaying when the engine could resume operation.
The Applicant has improved the state of the art by developing technologies that enable a cryogenic pump and vaporizer to supply natural gas from the liquid space to the engine when the vapor pressure drops below the threshold value. The cryogenic pump is actuated to pump LNG from the liquid space in the storage vessel through the vaporizer where it undergoes a phase change into either a supercritical or gas state. Upstream of a delivery line to the engine, an arrangement of check valves between a first supply line from the vapor space and a second supply line from the vaporizer allow the cryogenic pump to maintain the pressure in the delivery line above a predetermined value when the vapor pressure drops too low. This reduces downtime by allowing the engine to continue operating at least as long as there is sufficient LNG in the storage vessel.
In some applications it is known to use a hydraulic pump to drive the cryogenic pump that delivers LNG to the vaporizer. For example, the cryogenic pump can be a reciprocating piston-type pump which is driven by a double-acting piston in a cylinder of a hydraulic motor. A switchable valve directs hydraulic fluid from the hydraulic pump into and out of the cylinder in the hydraulic motor such that the double-acting piston reciprocates back and forth.
It is also known to employ hydraulic pumps that are directly driven by the engine. With these pumps the flow rate of hydraulic fluid is directly proportional to engine speed. Energy is wasted when unused hydraulic flow is recirculated in those regions of the engine map where engine speed is high but natural gas demand from the engine is low. For this reason it would be advantageous to decouple the direct relationship between hydraulic fluid flow rate and the speed of the internal combustion engine. This can be accomplished by employing a variable displacement hydraulic pump or an electrically driven hydraulic pump.
There are advantages to employing an electrically driven hydraulic pump when integrating a natural gas fuel system onto engines supplied by a variety of manufacturers. Both electrically driven and directly driven hydraulic pumps require plumbing for hydraulic fluid. Where the plumbing needs to be routed influences where the pumps can be placed. However, directly driven hydraulic pumps preferably need to be located close to or in line with a power take off from the engine due to the mechanical linkage required to drive the pump. In contrast the electrically driven hydraulic pump requires a wiring harness that supplies electrical power to the pump. The flexibility in routing the wiring harness allows the electrically driven hydraulic pump to be located such that the hydraulic plumbing can be simplified and to reduce the likelihood of having to modify the underlying engine. The complexity and cost of mounting electrically driven hydraulic pumps is reduced compared to directly driven hydraulic pumps.
While there are advantages associated with using one electrically driven hydraulic pump to supply the hydraulic fluid flow for the cryogenic pump, in some applications existing electrically driven hydraulic pumps cannot individually supply the maximum hydraulic fluid flow required to meet the maximum gas flow demand of the engine. This is one reason that has prevented electrically driven hydraulic pumps from being employed in the past.
A problem to be solved is how to control two or more hydraulic pumps supplying hydraulic fluid to a cryogenic pumping apparatus over the entire engine map and over the lifetime of each hydraulic pump. The state of the art is lacking in techniques for controlling such a hydraulic system.